Most adults in the U.S. now have cellphones and one in four are using smartphones. With their rich features and capabilities, these devices are driving the mobile app economy. As of June 2010, 59% of smartphone owners and nearly 9% of feature phone owners report having downloaded a mobile app in the last 30 days.

To better understand the growing popularity of mobile apps, The Nielsen Company launched the Mobile Apps Playbook in December 2009. The most recent version is now available. “The State of Mobile Apps,” features select highlights from the report and is based on an August 2010 survey of more than 4,000 mobile subscribers who had reported downloading a mobile app in the previous 30 days.

One of the main challenges facing apps publishers is making sure consumers “discover” their apps. According to Nielsen’s survey, searching application stores on their phones is the preferred way for discovering new apps for users of feature phones and smartphones alike (57% and 40%, respectively). The next most popular “preferred” form of app discovery is through the recommendation of a family member or friend. However, smartphone apps users are more likely than feature phone users to seek out other ways of discovering new mobile apps.

Most Americans can’t imagine leaving home without their mobile phones. Nearly all adults in the U.S. now have cellphones, with one in four having smartphones, pocket-sized devices more powerful than the computers initially used to send men to the moon. By the end of 2011, Nielsen predicts that the majority of mobile subscribers in the U.S. will have smartphones.

Cell phone apps may be endlessly fascinating to those of us here in Planet Tech. But in the real world — not so much.

About one-in-three adults has a cell phone with apps, The Pew Research Center’s Internet & American Life Project and The Nielsen Company found in recent surveys. Their new study, “The Rise of Apps Culture,” adds that just 68% of those who do have a phone with apps — about a quarter of all adults — actively use them.

Although interest in apps is growing, “this is the early stage of adoption when many cell owners do not know what their phone can do,” says Kristen Purcell, the Pew Internet Project’s associate director for research. “The apps market seems somewhat ahead of the majority of adult cell phone users.”